1. Field of the Invention
The present invention relates to an optical scanning device, and more particularly to an optical scanning device that is suitable for an image forming apparatus such as a digital copier or a laser printer.
2. Description of the Related Art
Image formation using an optical scanning technology is widely implemented in image forming apparatuses such as digital copiers and laser printers.
Examples of known optical scanning technologies for enabling high-speed image formation include a multiple-beam scanning technology. As a laser light source suitable for such a scanning technology, a vertical cavity surface emitting laser (VCSEL) has been increasingly used.
Another type of laser light source used in a multiple-beam scanning technology includes an edge emitting semiconductor laser array. An edge emitting semiconductor laser (hereinafter, the “edge emitting laser (EEL)”) may be used in plurality to employ a compound prism to combine beams. However, these technologies enable only several light-emitting elements to be arranged at the same time.
In contrast, the VCSEL enables tens to hundreds of laser light-emitting elements to be arrayed on the same plane from which laser beams are output, and each of the laser beams can be modulated individually. Accordingly, this technology enables tens to hundreds of scanning lines to be drawn simultaneously. Therefore, the VCSEL can fully achieve high-speed image formation, which is an advantage of the multiple-beam scanning.
However, one of the problems unique to a VCSEL is that the amount of light changes dynamically at the time the elements are driven (dynamic characteristics). Such dynamic characteristics include, for example, droop characteristic, rise time characteristic, and fall time characteristic (for example, see Japanese Patent Application Laid-open No. 2006-332142 and Japanese Patent Application Laid-open No. 2008-213246).
Known causes of these kinds of phenomena, observed in a general semiconductor laser, include a change in a threshold current caused by the light source element itself being heated by a current applied thereto, and a capacitor-resistor (CR) time constant of an electric circuit. Because of these phenomena, image density varies, resulting in poor image quality such as uneven density or uneven color tone. Therefore, a technology such as automatic power control (APC) has been used to reduce such variations.
An EEL and a VCSEL used conventionally exhibit different characteristics, such as wavelength characteristics or driving characteristics due to their structural differences.
In particular, an EEL and a VCSEL have significantly different driving characteristics. In an EEL, because mode hopping (wavelength hopping) occurs in an extremely short time period at the time the EEL is driven, heat can change the length of the optical path in the resonator. In addition, the gain function of a laser medium changes due to a sudden characteristic change that occurs immediately after the application of the current. When these changes occur, a light beam can jump into a mode that is most advantageous to oscillating (a mode with a large gain).
FIG. 1 is a schematic of an example of an observation of the mode hopping. The horizontal axis in FIG. 1 indicates the wavelength, and the vertical axis indicates elapsed time. FIG. 1 illustrates the optical response of each wavelength within a frame of approximately 50 nanoseconds, observed immediately after the application of the driving current.
Immediately after the driving current is applied, a short wavelength mode (648.17 nanometers) rises. Longer wavelength modes gradually come to dominate (mode hopping), and these modes eventually stabilize into a single mode.
In FIG. 1, the spacing between neighboring modes is 0.16 nanometer. The spacing of approximately 0.2 nanometer between neighboring modes is extremely smaller than that in a generally used EEL having a wavelength of 650 nanometers, making no problem in terms of image forming characteristics. In other words, the total optical output across the entire modes is relatively stable with respect to any change in internal conditions of the elements.
On the contrary, because a VCSEL has substantially only one wavelength, no mode hopping theoretically occurs in principle. In other words, the wavelengths of the neighboring modes are extremely remote to each other, e.g., the wavelength of a neighboring mode is half or twice the oscillating wavelength. For example, a mode, neighboring a VCSEL having a wavelength of 780 nanometers, has a wavelength of 390 nanometers or 1,560 nanometers. Because the wavelength difference is extremely large, such a mode is not caused to oscillate, being unable to attain the gain of the laser medium.
Therefore, because a VCSEL keeps oscillating in the same mode under any conditions, VCSELs are less flexible compared with EELs, and unable to achieve a stable optical output.
In addition, in a semiconductor laser, when a current is applied to suddenly change the temperature of the active layer, such a temperature change leads to a change in the refractive index. A change in the refractive index further induces a change in the optical confinement. Accordingly, a divergence angle (far field pattern (FFP)) of a laser beam instantaneously changes; the FFP is small near a current application time t to a current application time 0, and increases over time. In an optical system having an aperture, such a change is generally translated into a change in the amount of light (rise time characteristic) caused at the time the element is driven with a constant current.
In a scanning optical system, such a variation affects the amount of light on a target surface in a larger degree irrespective of whether the type of the system is an under-field type or an over-field type.
FIGS. 2A and 2B are graphs representing the dynamic characteristics of the unstable amount of light caused by combinations of the factors described above. The horizontal axis indicates the elapsed time since the time zero which is the time the current is applied, and the vertical axis indicates the observed amount of light. Such dynamic characteristics of the amount of light in the VCSEL are observed when a large current is applied. Examples of a situation in which a large current is applied include:
(1) when a part of a light beam is fed back to stabilize the optical output of the VCSEL, the amount of light in the beam travelling to the photosensitive element becomes less than a half; and
(2) when the photosensitive element is less sensitive, a larger current needs be applied.
In the region where a large current is applied, FIG. 2A indicates an example with a low optical intensity, and FIG. 2B indicates an example with a high optical intensity. The difference Δ between the amount of light (P2) at the time when the optical intensity temporarily surges and the amount of light (P1) near the current application time zero is Δ(a) percent and Δ(b) percent, indicated in FIGS. 2A and 2B, respectively. Δ is calculated using Equation 3 below:Δ=|(P1−P2)|/P2  (3)
If the difference between Δ(a) and Δ(b) is large, the amount of light varies depending on an optical scanning device, whereby a poor image is formed.
The problem of the dynamic characteristics of the amount of light illustrated in FIGS. 2A and 2B described above has conventionally been addressed by means of an electrical driving control technology, for example. However, to address the problem unique to the VCSEL, the above technology alone is inadequate.